Many industrial applications require plastic compounds which are flexible so that the plastic can tolerate various stresses without failing. For example, when a plastic adhesive is employed to bond materials having different thermal expansion coefficients and the assembly is subjected to thermal cycling, the plastic adhesive should be compliant enough to reduce the stresses in the bond joint by deforming so that high stresses are not introduced into the adherends. High modulus adhesives are known to crack brittle materials such as ceramics or to warp more deformable materials such as metals. Not only does the flexibility of a plastic adhesive serve to allow the adhesive itself to remain intact during such stress, but a flexible plastic adhesive also protects its adherends from cracking as well. In illustration, aluminum heat sinks are often bonded to silicon chips or ceramic printed wiring boards to dissipate heat. However, without a flexible bonding material, the bond itself is subject to failure as are the silicon and ceramic materials given the inherent thermal mismatch of such an assembly and the inevitable thermal cycling it will sustain.
Presently, the flexible polymer market offers a wide variety of flexible compounds, including such compounds as polyurethanes, polysulfides, and silicones. However, each of these types of plastics exhibit shortcomings in practice. Silicones and polysulfides are contaminating in many applications, are limited in diversity of formulation, and have low tensile strength, with polysulfides additionally having low tear resistance. Polyurethanes are subject to foaming and bubbling if moisture is present and have a limited upper temperature range. And none of these types of plastics is available as a solid resin, which would be suitable for use in a room-temperature stable film adhesive. Thus, a plastic compound without these attendant disadvantages would be desirable.
In comparison, epoxy-based compounds do not suffer from the above-described disadvantages attendant to polyurethanes, polysulfides, and silicones. Epoxy-based compounds are, in fact, available in solid form and offer diversity of formulation and sufficient tensile strength. Moreover, epoxy compounds demonstrate the ability to strongly adhere to a variety of materials, including metal, glass, plastic, wood, and fiber, and consequently are often used to bond dissimilar materials. Further, epoxy compounds are known to exhibit excellent resistance to attack by many corrosive chemicals. However, there are presently no commercially-available solid epoxy compounds that are at once flexible, conveniently stored, and readily curable.
Presently, epoxy-based compounds are available in two forms, namely, two-component systems or one-component systems, neither of which is both convenient to store and readily curable. Two-component epoxy-based compounds are readily curable at room temperature but are inconvenient to use and store. The components of two-component systems must be accurately measured, properly mixed, and degassed just prior to use. Thus, the various components to be mixed must be separately stored until use, and production workers are charged with the added responsibility of preparing epoxy-based adhesives having uniform properties. Not surprisingly, two-component epoxy-based compounds are not favored.
One-component epoxy-based compounds are available for industrial application in two basic forms: latent-cure epoxy compounds and frozen pre-mix, flexible epoxy compounds. These epoxy compounds are stored as a single component, requiring curing at elevated temperatures. Latent-cure epoxy-based compounds generally include Bisphenol-A resins and/or epoxy-novolacs. These rigid epoxies exhibit strong adhesion for many materials and may be conveniently stored at room temperature. However, rigid epoxy-based compounds form brittle bonds that are often insufficiently pliant for bonding dissimilar materials. For example, a brittle bond between dissimilar materials with different thermal expansion rates may be unable to withstand the stresses caused by the thermal mismatch, so that both the bond and its adherends may be susceptible to failure.
Frozen pre-mix, flexible epoxy-based compounds are also employed by industry in paste and film adhesives, although the usage of such adhesives is far surpassed by the usage of rigid epoxy adhesives. A description of frozen pre-mix, flexible epoxy adhesives is found in U.S. Pat. Ser. No. 4,866,108, assigned to the present assignee, which discloses and claims the composition behind Flexipoxy 100 Adhesive, a frozen flexible epoxy adhesive developed for spacecraft electronic applications. In comparison to rigid epoxy-based compounds, flexible epoxy adhesives form more pliable bonds that are capable of successfully adapting to stresses between dissimilar materials caused by differ- ing rates of expansion. However, in contrast to rigid epoxy-based compounds, frozen pre-mix flexible epoxy adhesives must be stored in a frozen state and must be thawed prior to use. Moreover, frozen adhesives offer a limited working life of only about 2 to 8 hours once thawed, whereas at least one week of working life is realistically required for general automated bonding operations. Therefore, frozen pre-mix, flexible epoxy adhesives in general, and frozen flexible epoxy film adhesives in particular, are widely considered impractical for use in high volume automated processing given the scheduling difficulties wrought by both the need to thaw the adhesives as well as the limited working life available after thawing.
Thus, a need remains for a one-component epoxy-based compound that is available in solid form and is sufficiently flexible such that it can withstand the rigors of varying expansion rates between bonded materials. Moreover, the epoxy-based compound should offer the convenience of room temperature storage and be readily curable.
These compounds are particularly suitable for use in room-temperature stable film adhesives.